Hail is produced by the types of cloud – cumulonimbus – that are associated with thunderstorms, and require a strong upward wind to form.

Drops of water are taken up and down throughout the thundercloud, freezing as they reach the top. They can stay there for a long time, gathering more and more ice, before getting too big for the updraught to hold them any longer and falling to the ground.

Stones anywhere between 5mm and 15cm can be the result.

People have been advised to move their cars under cover and away from trees because of the large stones in Australia.